The present invention relates to crystal oscillators, and more particularly, to controlling an output frequency of an oven controlled crystal oscillator in response to a monitored temperature sensitive parameter and a calculated relationship between the monitored parameter and a frequency compensating signal.
Oscillating circuits play a central and increasingly important role in digital and analog electronic systems. Digital devices require precise system timing, a function provided by oscillators and similar timing sources. Telecommunication and data transmission systems, which have analog and digital components, likewise rely on oscillators for modulation, demodulation, system clocking and other functions.
A standard choice for a highly stable frequency source in such applications is a crystal-based oscillator or resonator. Although atomic frequency standards are highly accurate, such devices are undesirable in most applications in view of cost and environmental capability.
While crystal oscillators are stable in comparison with non-crystal-based circuits, the crystal oscillators nevertheless exhibit a degree of variance in frequency and stability owing to the inherent frequency response to temperature changes and other environmental influences of crystals.
An approach to compensate for the temperature effects on a crystal resonator is to maintain a constant temperature of the temperature sensitive section of the oscillator. Such devices, oven controlled crystal oscillators (OCXO), provide enhanced stability and frequency control. However, OCXOs require increased manufacturing considerations and hence, greater costs so as to match the stabilities associated with their atomic frequency reference counterpart.
Double oven controlled crystal oscillators (DOCXO) provide the required stability needed for the higher stability applications. However, DOCXOs require increased material costs over the single oven.
The need exists for further controlling the frequency of an OCXO to provide enhanced stability over greater temperature ranges. In addition, the need exists for an economical method of controlling the frequency of a crystal oscillator without requiring a double oven construction.
The present invention provides an apparatus and method for the on-board adjustment of an output frequency of a crystal oscillator in conjunction with a heater and a frequency compensator, by monitoring a parameter corresponding to a power consumption of the heater and providing a frequency compensating signal to the input of the frequency compensator in response to the monitored parameter and a derived relationship between the monitored parameter and a required frequency compensating signal for the given crystal oscillator.
In a preferred configuration, the derived relationship is set forth in terms of the monitored parameter and a corresponding frequency compensating signal. This relationship can be a linear, non-linear or user defined relationship.
In one configuration, the monitored parameter corresponding to a power consumption of the heater is determined by the voltage measurement across a resister in series with the heater, where the varying voltage potential signifies a varying current consumption with a constant supply. However, it is understood a secondary relationship, or correspondence can be identified between the monitored parameter and the predetermined relationship.
Thus, one configuration an oven controlled crystal oscillator (xe2x80x9cOCXOxe2x80x9d) includes an on-board processor and associated memory to retain the relationship and calculate a frequency compensating signal as a function of the monitored parameter and the retained relationship. The present invention thereby provides for the on-board frequency error correction due to temperature influence on a crystal in an oven controlled crystal oscillator.